Abstract: To investigate the influence of the secondary air ratio on combustion and NOx emissions in a 700 MW ultra-supercritical circulating fluidized bed (CFB) boiler, a full-loop three-dimensional numerical model was established. Under the condition of a constant total air flow rate, a detailed comparative analysis was conducted on the in-furnace temperature field and pollutant formation characteristics at secondary air ratios of 55%, 60%, and 65%. The results show that when the secondary air ratio increases from 55% to 60%, the air–coal distribution in the furnace is optimized and the reducing atmosphere in the dense phase zone is improved; consequently, the NOx concentration at the furnace outlet decreases from 266 mg/Nm³ to 165 mg/Nm³, representing a reduction of 38%. However, as the secondary air ratio further increases to 65%, oxygen deficiency occurs in the dense phase zone, combustion deteriorates, and CO emissions rise sharply. Overall, it is concluded that a secondary air ratio of 60% is suitable for this operating condition, as it balances combustion efficiency with low NOx emissions.